The fight behind Mayweather vs. Canelo

On Saturday, undefeated boxers Floyd "Money" Mayweather Jr. and Saul "Canelo" Alvarez will square off at the MGM Grand in what is expected to be one of the biggest pay-per-view (PPV) fights in years—and maybe, some suggest, of all time.

In the event, promoted as "The One," Mayweather and Alvarez will compete for both a world championship title and pre-eminent status as boxing's brightest star.

But a battle just as big is being waged outside the ring, this one between CBS' Showtime and Time Warner's HBO.

A $300 million market

For years considered an also-ran in boxing PPV, Showtime pulled off a coup earlier this year when it signed Mayweather—whose bouts had previously been carried by HBO—to a multiple-fight contract. "The One" will be the second of the agreement.

Some say that the market's tides have turned in Showtime's favor, largely because of Mayweather, and that this Saturday could be a decisive milestone for the network in its bid to become the undisputed king of PPV boxing, a sports segment whose revenue varies year to year but can easily exceed $300 million. (That total is for boxing itself, which handily tops mixed martial arts on an annualized revenue basis, though with far fewer PPV events.)

"When Floyd Mayweather went to Showtime, a lot of the momentum shifted," said Richard Schaefer, CEO of Golden Boy Promotions, the promoter of Saturday's fight. "If you have the No. 1 star, then of course you have the momentum."

HBO declined a CNBC request for comment.

Schaefer said that Showtime hasn't worked with a star of Mayweather's stature since Mike Tyson, whose bouts include three of the five most-purchased PPV fights in history.

"After Tyson retired, they didn't really have the right ingredients to cook up a good meal," he said. "With Floyd Mayweather, they have the right ingredient and are in the process this year of becoming the largest PPV provider."

Although Tyson provided blockbusters for Showtime during his career, the largest pay-per-view event in history was 2007's megafight between Mayweather and Oscar de la Hoya. Carried by HBO, it generated more than 2.4 million buys and generated more than $130 million in revenue.

"Showtime has not been in the PPV business as much in recent times as HBO has," said Stephen Espinoza, general manager of Showtime Sports. But with Mayweather on board, he expressed cautious confidence in his network's progress.

"I don't think anyone can realistically, objectively say that the two networks aren't at least on par with each other," Espinoza said. "I wouldn't argue with anyone who says that we have pulled ahead."

Showtime's boxing resurgence has also been facilitated by CBS CEO Les Moonves, a boxing aficionado, he said.

"He is a huge boxing fan," Espinoza said. "He attends fights regularly in Los Angeles, big fights and small fights. I could not have done a deal of the scale that we did with Mayweather without his approval and enthusiasm."

Mayweather Promotions CEO Leonard Ellerbe said he feels that enthusiasm.

"Showtime and CBS made it very clear that they wanted to be in the Mayweather business," he said. "It's the best move that they could have made."

Ellerbe said that Mayweather's confidential deal with Showtime has vaulted the network into the PPV lead, and he predicted record-breaking gross revenue for Saturday's fight.

"HBO has been the premier network for so many years. Now Showtime has not only caught up with them, they've surpassed them," Ellerbe said. "We've got a good expectation of surpassing the gross that we did in the de la Hoya fight."

He declined to specify how much the fighter stands to make from the contest but called the contract "very lucrative." Various estimates put Mayweather's likely payout at more than $40 million.

Alvarez, a rising star with a huge, dedicated following of his own, also is set to receive a big payout.

While insiders point to Saturday as the consensus pick for the year's most anticipated boxing event, HBO will answer with two high-profile fights in October and November, one showcasing champion Juan Manuel Márquez, and the other featuring global superstar Manny Pacquiáo.